1. Field of the Invention
The present invention relates to timing circuits, and more particularly to precision, dithered phase, swept delay circuits. A particular application is radar timing circuits including precision swept delay circuits for equivalent time ranging systems. It can be used to generate a spread-spectrum swept-delay clock for sampling-type radar, laser and TDR systems, as well as radio and ultrasonic systems.
2. Description of Related Art
High-resolution pulse-echo systems such as wideband pulsed radar, pulsed laser rangefinders, and time domain reflectometers (TDR) generally sweep a timing circuit across a range of delays. The timing circuit controls a receiver sampling gate such that when an echo signal coincides with the temporal location of the sampling gate, a sampled echo signal is obtained in accordance with well-known equivalent time sampling techniques. The echo range is then determined from the timing circuit, so highly accurate swept timing is needed to obtain accurate range measurements. Unfortunately, highly accurate timing implies a very accurate and highly periodic radar pulse repetition frequency PRF, which creates an RF line spectrum. Narrowband or CW RF interference creates beat frequency interference when close to a spectral line. However, if there are no spectral lines due to spectrum spreading, then beat frequencies cannot occur.
One prior art approach to precision swept timing is a delay locked loop (DLL), wherein the delay between a transmit and receive clock is measured and controlled. Examples of DLL architectures are disclosed in U.S. Pat. No. 5,563,605, xe2x80x9cPrecision Digital Pulse Phase Generatorxe2x80x9d by McEwan, and in U.S. Pat. No. 6,055,287, xe2x80x9cPhase-Comparator-Less Delay Locked Loopxe2x80x9d by McEwan. Both of these DLL approaches employ a single highly periodic precision crystal oscillator as the time base. Neither patent discloses a means to spread the spectrum of a radar signal.
A second prior art approach to precision timing uses two oscillators with frequencies FT and FR that are offset by a small amount FTxe2x88x92FR=xcex94. In a radar application, a first oscillator at frequency FT triggers transmit RF pulses, and a second oscillator at frequency FR triggers a sampling gate for the echo RF pulses. Due to the small frequency difference xcex94, the timing of the sampling gate smoothly and linearly slips in phase (i.e., time) relative to the transmit clock such that one full cycle is slipped every 1/xcex94 seconds. The slow phase slip creates a time expansion effect of FT/xcex94 (xcx9c100,000 typically). Due to the expansion effect, events on a picosecond scale are converted to an easily measurable microsecond scale.
This two-oscillator technique was used in the 1960""s in precision time-interval counters with sub-nanosecond resolution, and appeared in a short-range radar in U.S. Pat. No. 4,132,991, xe2x80x9cMethod and Apparatus Utilizing Time-Expanded Pulse Sequences for Distance Measurement in a Radar,xe2x80x9d issued in 1979 to Wocher et al. Copending application, xe2x80x9cSelf Locking Dual Frequency Clock Systemxe2x80x9d Ser. No. 09/182,947, filed Apr. 1, 1999, now U.S. Pat. No. 6,373,428, by McEwan, and U.S. Pat. No. 6,072,427, xe2x80x9cPrecision Radar Time Base Using Harmonically Related Offset Oscillatorsxe2x80x9d by McEwan describe improvements to the two-oscillator technique. Both oscillators in a two-oscillator system are highly periodic precision crystal oscillators having a sharp line spectrum.
A common approach to spreading the line spectrum of a signal is to apply modulation to a steady clock signal after it is generated by a precision oscillator. Modulation formats include bi-phase and quad-phase, often at a high chip rate. U.S. Pat. No. 5,363,108 xe2x80x9cTime Domain Radio Transmission System,xe2x80x9d by Fullerton shows a clock followed by a stepped phase shifter. However, if the clock phase is uniformly stepped, rather than randomly stepped, a frequency shift will result without spreading the spectrum (serrodyne modulation).
While spread spectrum systems abound, particularly in communications, none address the area of phase dithered picosecond accurate single or dual clocks for sampling type radar systems. The prior art does not disclose a means to spread the spectrum of a precision oscillator itself.
The present invention is a randomly phase modulated precise clock system for pulsed radio, radar, laser, ultrasonic, and TDR ranging systems (and other timing applications which need an offset frequency) requiring high stability and accuracy, and a transmitter-receiver system incorporating the clock system. Several embodiments include but not limited to: (1) a single oscillator DLL system and (2) a two oscillator system. A fundamental problem solved is how to wideband modulate a crystal oscillator having a narrowband high Q resonator (the crystal) which exhibits a very narrowband characteristic (typical crystal bandwidth=100 Hz). Broadband modulation of the oscillator is accomplished by placing the crystal in a feedback path while injecting the broadband modulation in a forward path, where it appears as broadband phase modulation at the oscillator output. In the dual oscillator system, broadband phase modulation is applied equally to both transmit and receive oscillators so the deleterious effect of timing jitter introduced by the common phase modulation cancels.
A unique noise source is disclosed herein: a single CMOS oscillator is configured as a starved voltage oscillator to produce exceedingly noisy oscillations, which are used as a dither signal. In addition, the digital dither signal is wave-shaped and injected into the crystal oscillator(s) via a coupling network.
The majority of applications for the timing system described herein employ RF burst oscillators which must be phase locked, i.e., injection locked, to the clock signals. In other words, the RF oscillators must start with exactly the same RF phase on each repetition of the PRF clock. To aide in injection lockingxe2x80x94a form of shock excitationxe2x80x94negative emitter impedance is used to speed the edges of the clock signals.
A primary object of the present invention is to provide a high accuracy spread-spectrum swept timing circuit for time-of-flight ranging systems.
A further object of the present invention is to reduce interference to and from other RF systems sharing the same frequency band.
Yet another object of the present invention is to provide improved injection locking of RF oscillators needing precisely timed and phased startups.
Applications include low cost spread-spectrum radars for security alarms, home automation and lighting control, industrial and robotic controls, automatic toilet and faucet control, automatic door openers, fluid level sensing radars, imaging radars, vehicle backup and collision warning radars, and universal object/obstacle detection and ranging. One specific embodiment utilizing the present invention is a time domain reflectometer (TDR) where a pulse is propagated along a conductor or guidewire to reflect from a material for use in a variety of applications, such as an xe2x80x9celectronic dipstickxe2x80x9d for fluid level sensing. The single rod electronic dipstick (Goubau line) has an obvious need for spread spectrum timing since the rod is essentially an antenna and is thus susceptible to interference.